1. Field of the Invention
The present invention relates to a dispersion-compensating fiber which is employed to an optical transmission line including a dispersion-shifted fiber capable of long-haul, large-capacity optical communications utilizing wavelength division multiplexing (WDM) signals in a 1.5-xcexcm wavelength band or 1.6-xcexcm wavelength band and compensates for the dispersion of the dispersion-shifted fiber.
2. Related Background Art
In optical fiber transmission line networks used for high-speed, large-capacity communications over a long haul, the dispersion (chromatic dispersion) expressed by the sum of the material dispersion (dispersion caused by the wavelength dependence of refractive index inherent in the material of the optical fiber) and structural dispersion (dispersion caused by the wavelength dependence of the group velocity in the propagation mode) in the single-mode optical fiber employed as their transmission medium is a limiting factor for the transmission capacity. Namely, even when light outputted from a light source is assumed to have a single wavelength, it has an uniform spectrum width in the strict sense. When such an optical pulse propagates through a single-mode optical fiber having a predetermined chromatic dispersion characteristic, the pulse form may collapse since the propagation velocity varies among definite spectral components. This dispersion is expressed by a unit (ps/km/nm) of propagation delay time difference per unit spectrum width (nm) and unit optical fiber length (km). Also, it has been known that the material dispersion and structural dispersion cancel each other in the single-mode optical fiber, so that the dispersion becomes zero in the vicinity of 1.31 xcexcm.
A dispersion-shifted fiber is an optical fiber whose zero-dispersion wavelength has been shifted from a 1.3-xcexcm wavelength band to a 1.55-xcexcm wavelength band since the transmission loss of optical fibers becomes the lowest in the 1.55-xcexcm wavelength band; and a dispersion-compensating fiber is used as means for compensating for the dispersion of the dispersion-shifted fiber in general. As a technique for compensating for such a dispersion-shifted fiber, Japanese Patent Application Laid-Open No. HEI 10-39155 discloses a dispersion-compensating fiber and an optical transmission system using the same, for example.
Though being designed such that its dispersion becomes zero at a predetermined wavelength near the wavelength of 1.55 xcexcm, the dispersion-shifted fiber has a positive dispersion slope, thus being hard to suppress the occurrence of chromatic dispersion over the whole wavelength band in use. As a consequence, in communications based on the wavelength division multiplexing (WDM) system, which multiplexes light signals having wavelengths different from each other and thereby enables the transmission capacity to further increase, and the like, various transmission characteristics may deviate among wavelengths. For this matter, the above-mentioned dispersion-compensating fiber disclosed in Japanese Patent Application Laid-Open No. HEI 10-39155 is configured so as to improve the dispersion slope of the optical transmission line including the dispersion-shifted fiber as well.
Here, the dispersion slope is given by the gradient of a graph which indicates chromatic dispersion, and is expressed by a unit (ps/nm2/km).
The inventors have studied the conventional dispersion-compensating fiber and, as a result, have found problems as follows. Namely, the conventional dispersion-compensating fiber aimed at compensating for the dispersion of the dispersion-shifted fiber has a small chromatic dispersion, thereby necessitating a longer fiber length for compensating for the dispersion of the dispersion-shifted fiber. As a consequence, the efficiency of dispersion compensation has been low, and the apparatus and the like have inevitably increased their size. In addition, various characteristics of the conventional dispersion-compensating fiber applied to the dispersion-shifted fiber to be compensated for have not fully been optimized, and the dispersion slope in the optical transmission line to which the dispersion-shifted fiber is applied has not been improved sufficiently.
In order to overcome the foregoing problems, it is an object of the present invention to provide a dispersion-compensating fiber comprising a structure which compensates for the dispersion of a dispersion-shifted fiber, improves the total dispersion slope of an optical transmission system including the dispersion-shifted fiber over a wider wavelength band when applied thereto, and enables the dispersion to be compensated for efficiently and the apparatus to become smaller.
The dispersion-compensating fiber according to the present invention guides light signals of a 1.5-xcexcm wavelength band or 1.6-xcexcm wavelength band. It is an optical fiber for compensating for the dispersion of a dispersion-shifted fiber having a zero-dispersion wavelength in the 1.5-xcexcm wavelength band, and comprises a core region extending along a predetermined reference axis and a cladding region provided on the outer periphery of the core region. The core region is constituted by a core having an outside diameter 2a. The cladding region is constituted by a first cladding provided on the outer periphery of the core and having an outside diameter 2b; a second cladding provided on the outer periphery of the first cladding and having an outside diameter 2c; and a third cladding provided on the outer periphery of the second cladding.
In this dispersion-compensating fiber, the refractive index n1 of the core, the refractive index n2 of the first cladding, the refractive index n3 of the second cladding, and the refractive index n4 of the third cladding satisfy the condition of n1 greater than n3 greater than n4 greater than n2. Also, the respective relative refractive index differences xcex941 and xcex942 of the core and first cladding with respect to the third cladding satisfy at least the conditions of 1%xe2x89xa6xcex941xe2x89xa63%, and xcex942xe2x89xa6xe2x88x920.4%. The outside diameter 2a of the core and the outside diameter 2c of the second cladding preferably satisfy the condition of 2a/2cxe2x89xa60.3.
In this dispersion-compensating fiber, it is preferably that the relative refractive index difference xcex943 of the second cladding with respect to the third cladding satisfies the condition of xcex943xe2x89xa60.25%, further preferably xcex943xe2x89xa60.15%. Additionally, the outside diameter 2b of the first cladding and the outside diameter 2c of the second cladding preferably satisfy the condition of 2b/2cxe2x89xa60.3.
The dispersion-compensating fiber according to the present invention is characterized in that, when constituting an optical transmission system together with a dispersion-shifted fiber through which light signals of the 1.5-xcexcm wavelength band or 1.6-xcexcm wavelength band propagate, it has a length sufficient for the optical transmission system to yield a total dispersion slope of xe2x88x920.024 ps/nm2/km or more but 0.024 ps/nm2/km or less with respect to respective light signals having a shortest wavelength xcexS and a longest wavelength xcexL in signal wavelengths within the wavelength band in use.
Specifically, the dispersion-compensating fiber has a length LDCF which is set so as to satisfy the following condition with respect to light having a wavelength xcexm in signal wavelengths within the wavelength band in use:
|DDSF(xcexm)xc2x7LDSF+DDCF(xcexm)xc2x7LDCF|xe2x89xa6200 ps/nm
where
DDSF(xcexm) is the dispersion of the dispersion-shifted fiber at the wavelength xcexm;
LDSF is the length of the dispersion-shifted fiber;
DDCF(xcexm) is the dispersion of the dispersion-compensating fiber at the wavelength xcexm; and
LDCF is the length of the dispersion-compensating fiber.
More preferably, the length LDCF of the dispersion-compensating fiber is set so as to satisfy the following condition with respect to all signal wavelengths xcexall of light within the wavelength band in use:
xe2x80x83|DDSF(xcexall)xc2x7LDSF+DDCF(xcexall)xc2x7LDCF|xe2x89xa6200 ps/nm
where
DDSF(xcexall) is the dispersion of the dispersion-shifted fiber at all the wavelengths xcexall in use;
LDSF is the length of the dispersion-shifted fiber;
DDCF(xcexall) is the dispersion of the dispersion-compensating fiber at all the wavelengths xcexall in use; and
LDCF is the length of the dispersion-compensating fiber.
Also, the relative refractive index difference xcex941 of the core with respect to the third cladding preferably satisfies the condition of 1%xe2x89xa6xcex941xe2x89xa62% if a lower transmission loss is required, and preferably satisfies the condition of 2%xe2x89xa6xcex941xe2x89xa63% if a higher dispersion compensation efficiency is required due to a higher dispersion, and the range of xcex941 can be set appropriately in view of various conditions such as use, equipment, and the like.
Thus, the dispersion-compensating fiber comprising a core region having a single core and a cladding region provided on the outer periphery of the core region and having three claddings (a triple cladding structure) can improve the total dispersion slope in the optical transmission system to which the dispersion-compensating fiber is applied, if the refractive index and outside diameter of each part thereof are set so as to satisfy such conditions as those mentioned above. Also, when the dispersion-compensating fiber is formed into a module, it is possible to select a chromatic dispersion having an absolute value which is large enough to enable the dispersion to be compensated for efficiently and the apparatus to become smaller.
In particular, while the dispersion-shifted fiber has a large dispersion slope notwithstanding the fact that its dispersion has a small absolute value, the dispersion compensation effected by a dispersion-compensating fiber having a double cladding structure cannot compensate for the dispersion slope and the dispersion at the same time, or necessitates a length on a par with the dispersion-shifted fiber to be compensated for. Further, there are practical problems such as greater bending loss. When the triple cladding structure as mentioned above is employed in the cladding region, various characteristics of the dispersion-compensating fiber can be optimized for overcoming such problems.
For example, in the dispersion-compensating fiber having a triple cladding structure, the length of the dispersion-compensating fiber necessary for dispersion compensation can be made shorter as the absolute value of the dispersion having a negative value is greater, whereby the dispersion compensation can be made more efficient. In particular, the fact that the relative refractive index difference xcex941 of the core with respect to the third cladding is 1% or more but 3% or less is equivalent to the fact that, for example, the dispersion with respect to light having a wavelength of 1.55 xcexcm is about xe2x88x92200 ps/nm/km or more but 0 ps/nm/km or less.
Also, the fact that the relative refractive index difference xcex941 of the core with respect to the third cladding is 1% or more but 2% or less is equivalent to the fact that, for example, the dispersion with respect to light having a wavelength of 1.55 xcexcm is about xe2x88x92100 ps/nm/km or more but 0 ps/nm/km or less. Similarly, the fact that the relative refractive index difference xcex941 of the core with respect to the third cladding is 2% or more but 3% or less is equivalent to the fact that, for example, the dispersion with respect to light having a wavelength of 1.55 xcexcm about xe2x88x92200 ps/nm/km or more but xe2x88x92100 ps/nm/km or less.
Though the dispersion increases as the value of xcex941 is enhanced, it is necessary that, for example, the amount of GeO2 added to the core be increased in order to enhance the value of xcex941. On the other hand, an increase in the amount of addition of GeO2causes the transmission loss to increase. Therefore, it is preferred that, within the range of condition concerning xcex941, the condition of 1%xe2x89xa6xcex941xe2x89xa62% and the condition of 2%xe2x89xa6xcex941xe2x89xa63% be appropriately selected, in view of various conditions such as use, equipment, and the like, in the respective cases where a lower transmission loss and a higher dispersion compensation efficiency due to a higher dispersion are required.
Here, in the dispersion-compensating fiber according to the present invention, the relative refractive index difference xcex943 of the second cladding with respect to the third cladding preferably satisfies the condition of xcex943xe2x89xa70.1%. Also, the ratio of the outside diameter of the core to the outside diameter of the first cladding preferably satisfies the condition of 0.2xe2x89xa62a/2bxe2x89xa60.5. In addition, with respect to light having a wavelength of 1.55 xcexcm, the dispersion-compensating fiber according to the present invention has a bending loss of 10 dB/m or less at a diameter of 60 mm, a polarization mode dispersion of 0.5 psxc2x7kmxe2x88x92xc2xd or less, and a transmission loss of 1 dB/km or less.
In practice, there is a case where such a dispersion-compensating fiber is employed as a small-size module wound like a coil. In this case, in particular, lowering the bending loss enables the apparatus to reduce its size and suppress its accompanying increase in transmission loss.
When an optical transmission system is constituted by a dispersion-shifted fiber through which WDM signals of the 1.55-xcexcm wavelength band propagate and the dispersion-compensating fiber having a length sufficient for compensating for the dispersion of the dispersion-shifted fiber as mentioned in the foregoing, this optical transmission system yields a total dispersion slope of xe2x88x920.024 ps/nm2/km or more but 0.024 ps/nm2/km or less, preferably xe2x88x920.012 ps/nm2/km or more but 0.012 ps/nm2/km or less, with respect to respective light signals having the shortest wavelength xcexS and the longest wavelength xcexL in signal wavelengths within the wavelength band in use.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.